Basic Fourier-Transform NMR Spectroscopy In FT NMR (also called pulse- FT NMR) the signal is generated by a(90)rf pulse and then picked up by the receiver coil as a decaying oscillation with the spins resonance frequency o Generating an audio frequency signa The rf signal (o) from the receiver coil is"mixed"with an rf reference frequency oo(usually the same used to drive the transmitter ), resulting in an"audio signal"with frequencies Q2=0-Oo. The phase"of the receiver(x or y) is set electronically by using the appropriate phase for the reference frequency @o(usually, a complex signal -1.e, the x and y component is detected simultaneously by splitting the primary rf signal into two mixing stages with 90 phase shifted refernce frequencies The Analog-Digital Converter(ADC) For storage and processing the audio frequency signal has to be digitized first. There are two critical parameters involved dynamic range describes how fine the amplitude resolution is that can be achieved; usually 12 bit or 16 bit. 16 bit corresponds to a resolution of 1: 2(since the FID amplitudes will go from up to 2), meaning that features of the Fid smaller than /32768 of the maximum amplitude will be lost! 2. time resolution corresponds to the minimum dwell time that is needed to digitize a single data point(by loading the voltage into a capacitor and comparing it to voltages within the chosen dynamic range ). This needs longer for higher dynamic range, limiting the range of offset frequencies that can be properly detected(the sweep width sw High resolution spectrometers: dynamic range 16 bit, time resolution ca 6 us(133,333 Hz SW) Solid state spectrometers dynamic range 9 bit, time resolution ca. I us( 1,000,000 Hz sw) NYQUIST frequency: the highest frequency that can be correctly detected from digitized data, corersponding to two(complex) data points per period. After FT, the spectral width will go from NyQuist freq to +NyQUIST freq. Signals with absolute offset frequencies o larger than the NYQUIST freq. will appear at wrong places in the spectrum(folding)15 Basic Fourier-Transform NMR Spectroscopy In FT NMR (also called pulse-FT NMR) the signal is generated by a (90° ) rf pulse and then picked up by the receiver coil as a decaying oscillation with the spins resonance frequency w. Generating an audio frequency signal The rf signal (w) from the receiver coil is "mixed" with an rf reference frequency w0 (usually the same used to drive the transmitter), resulting in an "audio signal" with frequencies W = w - w0 . The "phase" of the receiver (x or y) is set electronically by using the appropriate phase for the reference frequency w0 (usually, a complex signal –i.e., the x and y component – is detected simultaneously by splitting the primary rf signal into two mixing stages with 90° phase shifted refernce frequencies. The Analog-Digital Converter (ADC) For storage and processing the audio frequency signal has to be digitized first. There are two critical parameters involved: 1. dynamic range describes how fine the amplitude resolution is that can be achieved; usually 12 bit or 16 bit. 16 bit corresponds to a resolution of 1:215 (since the FID amplitudes will go from -215 up to 215), meaning that features of the FID smaller than 1 /32768 of the maximum amplitude will be lost! 2. time resolution corresponds to the minimum dwell time that is needed to digitize a single data point (by loading the voltage into a capacitor and comparing it to voltages within the chosen dynamic range). This needs longer for higher dynamic range, limiting the range of offset frequencies that can be properly detected (the sweep width SW). High resolution spectrometers: dynamic range 16 bit, time resolution ca. 6 ms (= 133,333 Hz SW) Solid state spectrometers: dynamic range 9 bit, time resolution ca. 1 ms (= 1,000,000 Hz SW) NYQUIST frequency: the highest frequency that can be correctly detected from digitized data, corersponding to two (complex) data points per period. After FT, the spectral width will go from -NYQUIST freq. to +NYQUIST freq.. Signals with absolute offset frequencies w larger than the NYQUIST freq. will appear at wrong places in the spectrum (folding):